There exists an ever increasing demand for oil and gas production from offshore deep water sites. However, difficulty arises in bringing long prefabricated structures to a site, providing anchors at a desired seabed location, and anchoring the structures at great depth. Additionally, a deep water offshore platform must be able to tolerate the full range of conditions likely to be encountered at the site, including severe winds, waves, and currents. An effective anchoring system for a deep water drilling unit must be capable of restraining the structure to the drill site while also preventing excessive heave, pitch, and roll in high wind and strong waves conditions. This last objective is complicated by the fact that while anchoring systems restrain the platform from moving in calm seas, they also form an elastic system with a resonant frequency that can be within the frequency of wave motions encountered in deep ocean waters. An anchoring system that restrains a platform in calm seas can result in unacceptable heaving, pitching, and rolling of the platform in rough seas or high winds.
While several deep water platforms designs have been proposed and developed that permit drilling in deep offshore waters, existing designs are generally expensive to transport, difficult to secure to the seabed, and difficult to relocate. Moreover, some of the current designs are also prone to the suspension of drilling operations due to heave, pitch, and roll motions by high winds and waves. These problems greatly increase the cost of drilling exploratory and development wells in deep water sites.
The drilling and test production of deep water oil and gas wells is often achieved from a work deck supported atop a buoyant structure that is semi-submerged. These structures, however, still require that the buoyant structure be fixedly anchored to the seabed. For example, the tension leg platform uses steel cables or tubes anchored in the sea floor to hold in place a semi-submersible platform. The "Spar" approach uses multiple anchor lines, secured to the sea floor, to hold a semi-submerged caisson in place. Other proposed approaches for deep drilling include the use of flexible piles and tendons secured to the sea floor to anchor a floating platform in place. However, all of these approaches have the shortcoming that the platform cannot be easily relocated to new sites in addition to the fact that they all use expensive means to secure the platform to the seabed.
Therefore, one objective of the present invention is a mobile drilling unit that is relatively easy and inexpensive to construct and that can be economically relocated to other sites for cost-effective drilling.
Another objective of the present invention is to provide a mobile offshore deep water platform with a desirable motion response (drift, heave, pitch, and roll) in rough seas to permit drilling even under the conditions of high seas, swift currents, or strong winds.